This invention relates to a method of evacuating and sealing the vacuum chamber of a vacuum bottle and, more particularly, a method of evacuating and sealing such chambers in metal vacuum bottles having reflective surfaces on the vacuum chamber walls.
Vacuum bottles customarily include a first bottle spaced from a second bottle with only minimal contact therebetween in order to reduce conductive heat loss. The outer surface of the inner bottle and/or the inner surface of the outer bottle are usually coated with a reflective film or membrane so that the thusly "reflectorized" or "silvered" surfaces will cause the reflection of radiation and reduce radiation heat loss. Additionally, the chamber between the two bottles is evacuated in order to reduce gas conduction to or from the contents of the inner bottle.
The inner and outer bottles are often made of stainless steel which is sometimes nickel-plated prior to application of the reflectorized layer such as by the vapor-deposition of silver. The bottle is then heated in a furnace for one to two hours at a temperature of about 600 degrees centigrade in a furnace that is evacuated to about 10.sup.-4 -10.sup.-5 torr. In this manner, the bottle's vacuum chamber surfaces are "out-gased" and the vacuum chamber is evacuated to near the vacuum of the furnace itself. In this respect, a 20 mm-30 mm hole is conventionally cut into the outer bottle during the heating and evacuating process in order to permit the gases in the bottle's vacuum chamber to be drawn out of the chamber and through the evacuated furnace.
After the chamber is evacuated the hole in the outer bottle must be sealed. Such sealing, however, customarily includes a relatively high-temperature soldering step at as much as 700.degree. C.-800.degree. C., which tends to damage the reflectorized surfaces. In this regard, silver has relatively poor heat resistance and its reflective and adhesive abilities begin to deteriorate at temperatures of between 600.degree. C. and 650.degree. C. Accordingly, particularly if it is a silver film that is deposited on the bottle's evacuation chamber surfaces, the resulting damage reduces the bottle's ability to prevent radiation heat loss. It is an object of this invention, therefore, to provide a method of sealing the vacuum chamber in a manner which reduces damage to the reflectorized surfaces.
Conventional methods described above require strict control of the duration and temperature of the soldering step. In this regard, flux cannot normally be used when soldering in a vacuum; and, the conventional 20 mm-30 mm openings provide a relatively large surface that must be soldered. Consequently, the above-described method requires closely-controlled, time-consuming operations. Advantages of the method of the instant invention, however, result in a more rapid operation that need not be as closely controlled. Hence, the method of the invention is more efficient and less costly.
Another conventional method of sealing the evacuation chamber includes the use of a pinch-pipe for connecting the vacuum chamber to a source of vacuum external of the furnace. In this method the pinch-pipe is pressed or otherwise pinched closed after evacuation and then cut off outside of the furnace prior to being sealed with solder. In this method, a flux can be used and the soldering temperature can be as high as 700.degree. C. without too much damage to the silvered surfaces. The pipe, however, extends outwardly from the bottom of the bottle and must be covered in order to protect the seal and give the bottle a stable surface. This, however, not only increases the cost of the vessel, but makes it taller and more bulky so as to reduce the percentage of the overall structure that can be used for storage of contents. Another advantage of the instant invention, therefore, is that pinch-pipes are not required and the costly, bulky pinch-pipe covers are also eliminated.